Heart The heart is a muscular organ whose interior is divided into two pairs of chambers, one on the right, the other on the left; the chambers of each pair are connected with each other by a valve. Lying in the mid-left section of the chest, close to the breastbone, the human heart weighs about 12 ounces, beats 70 to 80 times a minute, and is enclosed by a sturdy membrane called pericardium. Its chambers are lined by a delicate membrane, the endocardium, and its vigorous muscular and connective tissues are nourished by the heart's own blood vessels, the coronary vessels. How the Heart Works This remarkable muscle serves as a pump controlling the blood stream in two circuits, the pulmonary and the systemic. The right side of the heart receives the blood from the large veins that drain the systemic circuit and propels it into the lungs where carbon dioxide is removed and oxygen is picked up. The oxygenated blood, collecting in the pulmonary veins, than enters the left side of the heart, from which it is pumped out again into the systemic circulation by way of the body's largest blood vessel, the aorta. The rhythmic pumping is in the form of a repeated contraction ( systole ) and relaxation (diastole). Every 60 seconds, this precisely adjusted human pump drives about five quarts of blood through the body. Auricles and Ventricles The four chambers of the heart have special roles in the pumping process. The upper chambers are called the auricles; the lower chambers, the ventricles. The auricle and ventricle on each side together form an independent part of the heart, somewhat like a duplex apartment; in effect, they make up a "right heart" and a "left heart". There is no connection for the blood into the pulmonary circuit, the left into the general body circuit. Valves of the heart Between the right auricle and right ventricle is a valve, called the tricuspid valve. Similarly, the left auricle and left ventricle are connected by the mitral valve, so named because of its apparent resemblance to a bishop's miter or tall cap. The sounds of the valves opening and closing are heard by the doctor when he listens with his stethoscope. In addition to the valves between auricle and ventricle on each side of the heart, there are valves at the blood's exit points: the pulmonary valve opening from the right ventricle into the pulmonary artery, and the aortic valve opening from the left ventricle into the aorta. All these valves, both within the heart and leading out of it, open shut in such a way is to keep the blood flowing only in one direction through the heart's two separate pairs of chambers: from auricle to ventricle and out through its appropriate artery. A Single Pumping Action Although the right and left sides of the heart serve two separate branches of the circulation, each with its distinct function, they are co-ordinated so that the heart efficiently serves both sides with a single pumping action. The valve action on both sides is also co-ordinated with the two phases of the pumping action. Thus, during the diastole, or relaxation phase, the oxygen-poor blood which was accumulated in the right auricle returning from the systemic or body circulation pours into the right ventricle. At the same time, the oxygen-rich blood which was accumulated in the left auriclereturning from the pulmonary circulation pours into the left ventricle. The weak walls of both auricles contract to press the blood into the relaxed ventricles. In the next or contraction phase, the systole, the valve between auricle and ventricle on each side closes, and the muscular walls contract the ventricles and sweep the blood through each passage into the pulmonary artery and the aorta. At the end of the contraction the pulmonary and aortic valves snap shut, preventing any backward surge of the blood to the ventricles. The diastole follows, the ventricles again fill with the flow from their separate auricles and the cycle is repeated. This co-ordinated rhythmic action goes on tirelessly day and night throughout every individual's lifetime. The Valve Cusps The valves, which must withstand considerable pressure, are composed of a special type of tough tissue. The mitral valve, between the left auricle and ventricle, has two cusps or leaflets. The tricuspid valve, between the right auricle and ventricle, has three cusps. Both valves function in the same manner. When blood pressure in auricle is higher than in the ventricle, the valve leaflets are swept open; as the blood flows downward, the auricular contraction at the start of the heart beat helps to push the blood along. As the blood fills the ventricle, the leaflets close, and with the contraction of the ventricle, pressure tightly shuts the valve. The valve leaflets are bolstered from below by a set of tough tendons with muscular attachments, enabling the leaflets to withstand the pressure and keep the valves from opening inward into the auricle. The two valves which control the exit passages from the heart, the aortic valve and the pulmonary valve, have three leaflets each, and they also only for one-way flow. Other valves at special stations along the line in the circulatory system keep the blood from pooling in the lower extremities of the body. Heart Disease. Heart disease, the leading cause of death, is a term covering a variety of more than 20 different diseases of the heart and blood vessels. The most common of these are rheumatic heart disease, hypertension or high blood pressure, and coronary artery disease. Other forms of heart disease can be caused by congenital malformations of the heart and major vessels, syphilis, diphtheria, abnormal functioning of the thyroid gland, or diseases resulting from vitamin deficiencies. Although approximately ten million Americans have some form of heart disease, the tremendous advances made by medical science have made it possible to treat and control these illnesses with increasing success. The majority of individuals who suffer a heart attack recover, and recurrent attacks of rheumatic fever which injure the heart can now frequently be prevented. ............